The Cushendun Formation, also known as the Cushendun Conglomerate, is a geologic formation of Early Devonian age in Northern Ireland where it outcrops at Cushendun, County Antrim.
History Surveys of Cushendun Bay were undertaken in 1886 and it was concluded that the rocks were metamorphic in origin.Cushendall, Sheet 14 Geological Survey of Ireland, 1886 The first geological maps of the area were produced by Bailey & McCallien (1934),Bailey, E.B. and McCallien, W.J. (1934): The metamorphic rocks of North-east Antrim. Transactions of the Royal Society of Edinburgh: Earth Sciences, vol. 58, pp.163-177 and Wilson (1953) was the first to conclude the rocks were Early Devonian in age and he placed them into the Old Red Sandstone as a new, unnamed member.Wilson, H. E. (1953): The petrography of the Old Red Sandstone rocks of the north of Ireland. Proceedings of the Royal Irish Academy, vol. 55, pp.283-320
Simon (1984b) defined the Cushendun Formation as a separate geological formation then known as the "Cushendun Conglomerate".
Geology and location
The formation is formed of thin breccia clasts overlain by conglomerate and quartzite. The age of the formation can not be dated any more precisely than the Early Devonian due to the lack of fossils preserved, and it was part of the Cross Slieve Group in the Lower Old Red Sandstone. It overlies the Precambrian age Dalradian Supergroup at its base through an unconformity, and it underlies the Early Devonian age Ballyagan Formation.Simon, J B. (1984b). Provenance and depositional history of the Lower Old Red Sandstone of northeast Antrim. Irish Journal of Earth Sciences 6, 213-228. The formation is also up to thick in places and has clasts of up to long.
The Cushendun Formation outcrops only at Cushendun Bay and the nearby Cushendun Caves (which formed the current cave system around 100,000 years ago) on the northeastern section of Cross Slieve; the southeastern section is dominated by the Cushendall Formation.
Paleogeography It is believed that the Cushendun Formation represented a desert environment where the larger rocks were deposited by a braided river stream.
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